In this article we will discuss about:- 1. Occurrence and Distribution of Hymenophylum 2. Sporophyte of Hymenophylum 3. Gametophyte 4. Phylogeny.
Occurrence and Distribution of Hymenophylum:
Hymenophyllum is distributed in humid, tropical and warm temperate regions of the world. It is characteristically found in tropical rain forests. In the west land forests of New Zealand it is especially abundant. It usually grows on the forest floor or on the wet branches of trees as epiphytes.
In Karnataka State Hymenophyllum is available in Shiradi Ghats near Sakleshpur (Hassan District), where they grow on wet trunks of trees. The number of species in the genus is variable. It has about 200-230 species. Some of the common species are H. australe, H.multifidum and H.recurvum etc.
Sporophyte of Hymenophylum:
Morphology of the Plant:
The plant body rarely exceeds 8-10 cm in sized. But in H.pulcherrimum the pendant fronds may be 0.5-1 metre feet in length. It has a rhizome which is very slender and extensive.
Branching of the rhizome is axillary. Roots are adventitious (lacking in some smaller forms) and are produced in pairs at the base of each leaf. But sometimes roots may be seen between the leaves. This is due to the failure of leaf primordium to develop above the region where the roots are produced.
The leaves of Hymenophyllum are very characteristic true to the name filmy ferns and are objects of beauty. The transluscent leaves when growing in close mats may easily be mistaken for bryophytic thalli.
Leaves are produced in acropetalous succession and show circinate vernation. But it is not uncommon to find sometimes, the unfolding of a young leaf between two mature leaves. This shows that many leaf primordia may remain dormant and may resume growth at a later stage.
Usually in H. australe, one of out six primordia grows further. The leaf primordia are formed in double series on the rhizome. The leaves have a flattened petiole. The lamina may be simple (H.cruenta) or dissected into unequal dichotomies as in the majority of species (Fig.144).
It is mostly one celled thick and has an open dichatomous venation. In each shank of dichotomy there is single vein. In H. cruenta there are a series of veins reaching the margin of the simple lamina. Sori are borne along the margin (Fig. 146a) or at the apex of each dichotomy. The leaves are pale green in colour and together with the roots they also seem to take part in absorption.
A transverse section shows a relatively narrow cortex surrounding a central stele and being surrounded by an epidermis. The cortex may be wholly sclerotic or partly thin walled (outer cortex) and partly thick walled (inner cortex). Vascular cylinder is protostelic even in the mature plant, a feature unusual to ferns, but not unusual in the background of its habitat. There is an outer endodermis surrounding the vasculature (Fig. 145a).
Internal to the endodermis is the pericycle one or many cells thick. Phloem surrounds the xylem but the two are separated by conjunctive parenchyma. The architecture of the xylem is variable in different species.
In H. demissum there is a ring of metaxylem surrounding a parenchymatous region in which is embedded the protoxylem. In H. scabrum the metaxylem ring is broken at two points to form two arcs, in H. cruenta one of the arcs (ventral) may be absent.
Leaf traces that depart from the vascular cylinder do not leave any gap. Branch traces are connected to the leaf traces or more often arise from it. Root: There is no root cap, the vascular cylinder is protostelic as in other leptosporangiatae. Xylem may be monarch or diarch.
Leaf blade is one cell in thickness (Fig. 145b) towards the margin but many celled thick towards the central vein. In H.dilatum leaf blade also is many celled thick. The vein consists of poorly developed xylem and phloem.
The apical growth of the leaf and rhizome is from a single apical cell.
Vegetative propagation may be brought about by the fragmentation of the rhizome when each portion grows into a new individual.
Spore Producing Organs:
The characteristic method of reproduction of the sporophyte is by the spore formation. Sori are developed singly at the tip of each dichotomy (Fig. 146a). In a dissected leaf blade sori are terminal and in an ensure blade they are marginal.
The sorus has a fertile tissue called ‘receptacle’ from which sporangia are produced. Receptacle development on the lamina is associated with the development of two flaps of tissue from the axial and abaxial surface of the leaf (Fig.146b).
This is the two lipped inducium. It is a true inducium as it does not represent the incurring margin of the lamina. The inducium over arches the sorus, offering protection to it. The sporangia develop in a basipetalous succession (Fig.146b) true to the gradate nature of the sorus.
Development of Sporancia:
Sporangial initials are first differentiated towards the apex of the receptacle. Succeeding ones appear in basipetalous fashion. Early stages of sporangial development resemble those of Osmundaceae.
A sporangial initial (leptosporangiate type) functioning like an apical cell cuts off one or two stalk cells and then divides periclinally to form an inner primary archesporial cell and an outer primary wall cell. The latter by undergoing only anticlinal divisions builds up the single layered wall, while the former produces the archesporium, the cells of which form the spore mother cells.
These divide meiotically to produce 256-512 haploid spores per sporangium. All spores are of the same type. The dehiscence of the sporangium is brought about by an obliquely vertical annulus (Fig.146c). Splitting of the sporangium is transverse. Spores are wind dispersed.
Gametophyte of Hymenophylum:
Structure and Germination of the Spore:
Spores have a triradiate shape because of the tetrahedral arrangement. They are minute in size and have a two layered wall. Early stages of germination takes place in situ. The spores divide into three radiately arranged cells (Fig.147a), when they are still within the sporangia. Further development takes place after shedding.
Each of the three cells divides transversely resulting in a six celled germling (Fig.147b). Further growth is independent in the three arms. Very soon growth ceases in two arms by the transformation of their terminal cell into a rhizoid. In the third arm an apical cell is established with two cutting faces. This is later replaced by a transverse row of cells.
A mature gametophyte of Hymenophyllum is filamentous, ribbon like and dichotomously branched or lobed. Rhizoids are produced from the ventral surface. Some of the branches of the gametophyte may grow erect.
Sex organs are mostly formed on the ventral side. Antheridia are borne near the margins of the branches. They are scattered all over the thallus. A mature antheridium has a single layered jacket (Fig. 147f). There is a triangular opercular cell at the apex to facilitate the exit of the antherozoids.
Archegonia are borne on short upright, special branches along the margin. Their development is typical of other ferns (see Opliioglossum). The neck is six to nine cells in height. The axial row consists of a bi-nucleate neck canal cell, a venter canal cell and an egg cell (Fig.147e).
Nothing is known about the embryo development in Hylmenophyllum. The only member where few details are available is Cardio manes. The first division is transverse (unlike in other leptosporangiates). Embryonal parts are differentiated only at a later stage. The orientation of the embiyonal organs is typical to leptosporangiate forms.
Phytogeny of Hymenophylum:
Because of their extreme simplicity, the filmy ferns are often regarded as the most primitive among ferns. In many features Hymenophyllum resembles the thalloid liverworts. In fact, in some older classifications, filmy ferns are placed in a group Bryopterids, and thought to be a link between bryophytes and pteridophytes.
Filmy nature of the leaves, poorly developed roots with no root caps, simple vascular organisation, all point towards a very simple nature of the plant-body. But this simplicity is due to reduction as a result of adaptation to a particular ecological habitat and not due to primitiveness. The leptosporangiate type of development certainly suggests a higher rank to Hymenophyllum among ferns.